1. Field of the Invention
The present invention relates to a developing device that uses a two-component developer containing a magnetic carrier and toner, and that can develop, without contact, an electrostatic latent image on an image carrying member with charged toner alone held by a development roller, and the invention also relates to an image forming apparatus provided with such a developing device, such as a copying machine, facsimile, or printer.
2. Description of Related Art
Conventionally, as a developing system using dry toner in an image forming apparatus employing an electrophotographic process, there are known one-component development that does not use a carrier and two-component development that uses a two-component developer, which charges nonmagnetic toner by use of a magnetic carrier, to develop an electrostatic latent image on an electrostatic latent image carrying member (photoconductor) by means of a magnetic brush formed by toner and a carrier on a development roller.
One-component development is suitable for obtaining high-quality images since the electrostatic latent image on the electrostatic latent image carrying member is not disturbed by a magnetic brush. On the other hand, since an elastic control blade controls the toner layer thickness on the development roller and also charges the toner, the toner adheres to the control blade, and thus nonuniform layer formation and hence image defects may result. Moreover, it is difficult to keep stable charging of the toner.
A one drum color superimposing system in which a plurality of color images are successively formed on a photoconductor is also developed; by superimposing toners precisely on the photoconductor, it is possible to form color images with less color displacement and thereby to obtain high-quality color images. Furthermore, in these days, a tandem system is developed that uses a plurality of photoconductors corresponding to the colors of toners, that forms color images synchronously with the conveyance of a transfer member on the photoconductors, and that superimposes color on the transfer member.
The tandem system, though excellent in high-speed operation, requires electrophotographic process members for different colors to be disposed side by side, and thus may lead to an increased size of the apparatus. To avoid such an increase in size, there is proposed a tandem type image forming apparatus in which an image forming unit, which is made compact by narrowing the intervals between photoconductors, is disposed. In a case of color printing where colors are superimposed in such a way, color toners need to be transmissive, and thus need to be nonmagnetic toners.
Thus, in a full color image forming apparatus, two-component development that charges and conveys toner by use of a carrier is typically employed. However, although two-component development can keep a stable charge amount for an extended period and is suitable for prolonging toner life, the magnetic brush mentioned above may affect image quality.
As a means to solve these problems, a developing system is proposed in which a developer is passed by use of a magnetic roller onto a development roller disposed without contact with a photoconductor, so that toner is transferred to this development roller to form a thin layer of nonmagnetic toner, and a toner image is formed by making the toner fly onto a latent image on the photoconductor under an AC electric field.
With this technology, since two-component development as described above is employed in a toner charging area, with toner long-life taken into consideration, and one-component development that make toner alone fly without contact with the photoconductor is employed in a subsequent developing area, with a view to enhancing image quality, it is possible to make use of the respective advantages of one-component development and two-component development.
However, in such a developing system, development failures such as image density failures and uneven images may occur. Thus, with attention paid to the particle size distribution of toner inside the developing device, a method is proposed that prevents degradation in developing performance and image quality.
For example, in JP-A-H6-295123, a method of adjusting toner particle size inside a developing device is proposed that includes: a first step in which nonmagnetic toner is transferred from a magnetic roll to a development roll, under application of a bias between the magnetic roll and the development roll, to form a toner layer on the development roll; a second step in which toner having large particle size and easy to transfer is returned from the toner layer on the development roll to the magnetic roll under application of a bias in a direction opposite to that in the first step; and a third step in which toner having small particle size remaining on the development roll is transferred to an electrostatic latent image carrying member under application of a bias between the development roll and the electrostatic latent image carrying member.
In this way, it is possible to remove toner having large particle size from the toner layer on the development roll, to form a toner layer containing toner having relatively small particle size alone on the surface of the development roll, and to remove the toner having small particle size remaining on the development roller after transferring it to an electrostatic latent image carrying member (photoconductor). Thus, it is possible to keep the particle size distribution of toner inside the developing device substantially even, and to keep the initial developing performance and thereby to prevent degradation in image quality.